1. Field of the Invention
The present invention relates to an overcurrent protection device for a power supply device and related power supply device, and more particularly, to an overcurrent protection device and related power supply device capable of achieving an identical voltage for the overcurrent limit corresponding to each input voltage.
2. Description of the Prior Art
Power supplies are utilized for supplying electrical energy for electronic devices, and can be generally divided into linear power supplies and switching power supplies. Compared to the linear power supplies, the switching power supplies have advantages of smaller size, lighter weight, and greater efficiency, so as to be widely applied to different areas, such as mobile communication devices, personal digital assistants, computers and related peripheral devices, servers, and network devices.
Protection schemes, such as overvoltage protection, overcurrent protection, or overpower protection, etc., play a very important role in a control circuit of a power supply for safe operation of the power supply. Power supplies that have comprehensive protection schemes can prevent internal elements and related devices from being damaged under current overload or short circuit conditions.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a power supply 100 in the prior art. The power supply 100 includes a transformer 102, a power switch 104, a current sensing unit 106, a comparator 108, and a pulse width modulation control unit 110. The transformer 102 includes a primary side circuit L1 and a secondary side circuit L2 for transforming an input signal VIN into an output signal VOUT. The power switch 104 is coupled to the primary side circuit L1 for controlling operations of the transformer 102. As shown in FIG. 1, the power switch 104 is implemented by a power transistor Q1. The pulse width modulation control unit 110 is utilized for controlling on/off status of the power switch 104 by outputting a control signal. The current sensing unit 106 is coupled to the drain of the power transistor Q1, and implemented by a current sensing resistor RCS for providing a current sensing signal VCS in order to detect current IL1 passing through the primary side circuit of the power transistor Q1. The comparator 108 is utilized for comparing the current sense signal VCS with a reference voltage VREF in order to provide a result for the pulse width modulation control unit 110 to determine whether the overcurrent condition exists. For example, when the current sensing signal VCS is greater than the reference voltage VREF, the comparator 108 can pass an indication signal SOC to the pulse width modulation control unit 110. The pulse width modulation control unit 110 then enables to turn off the power transistor Q1 in order to reduce the current IL1 passing through the primary side circuit.
The above protection scheme can keep the current within a proper range by comparing the current sensing signal VCS with the reference voltage VREF. However, when the current sensing signal VCS is greater than the reference voltage VREF, the power switch 104 can not turn off immediately due to non-ideal factors. Actually, the pulse width modulation control unit 110 may enable to turn off the power switch 104 after a non-ideal delay. As a result, since the overcurrent condition exists until the power switch 104 actually turns off, there exists a propagation delay time Tdelay in which the current will be greater than a predetermined value. In other words, a voltage of actual initial overcurrent protection (protection voltage) is usually greater than a voltage corresponding to occurrence of the overcurrent condition (i.e. VREF), and the protection voltages will be different for each input voltage VIN.
In detail, FIG. 2 is a schematic diagram of protection voltage difference for different input voltages due to propagation delay. The input signal VIN of the power supply 100 is proportional to the rising slope of the current sensing signal VCS. Therefore, a high input voltage VH will generate a current sensing signal with greater slope and a low input voltage VL will generate a current sensing signal with less slope. The reference voltage is VREF. Moreover, there is a same propagation delay time Tdelay in the same power supply. The propagation delay time Tdelay is irrelevant to the input signal VIN. As shown is FIG. 2, as the current sensing signal VCS rises to a power limit level of the reference voltage VREF, the comparator 108 passes an indication signal SOC to the pulse width modulation control unit 110 so as to turn off the power transistor Q1. After a propagation delay time Tdelay during which the power switch 104 is turned off, the current IL1 passing through the primary side circuit is disabled. As shown in FIG. 2, since the overcurrent condition exists until the power switch 104 actually turns off, the input signal continues to provide power, so that the high input voltage VH has a corresponding protection voltage VOPPH and the low input voltage VL has a corresponding protection voltage VOPPL. Therefore, the protection voltage will greater than the reference voltage VREF, and the difference increases as the input signal becomes greater. In such a condition, when the power operates over a wide range (AC input voltage ranges from 90 Vac to 264 Vac), the protection voltage may vary obviously, and the output powers for the high input voltage and the low input voltage will be largely distinct from each other.